1. Field of the Invention
The invention relates to the synthesis of hexafluoropropylene from tetrafluoroethylene.
2. Description of Related Art
Hexafluoropropylene (HFP) is a well known fluoromonomer used for copolymerization with other fluoromonomers to form fluoropolymers such as tetrafluoroethylene(TFE)/HFP copolymer commonly know as FEP. The principal method of making HFP is by the pyrolysis of TFE. This pyrolysis reaction is disclosed in U.S. Pat. No. 2,758,138 as follows:C2F4→2CF2C2F4+CF2→C3F6 (HFP)As disclosed in '138, the pyrolysis reaction is carried out by feeding TFE into a reaction zone described as a reaction tube at a temperature of 750° C. to 900° C. and at certain feed rate, and reduced pressure conditions to obtain yields above 75%. The tubular reactor is made of or lined with alloy steel or other high temperature resistant material which is substantially inert to the reaction products. The Examples use a small stainless steel reaction tube ⅜ in (0.95 cm) in diameter. In an earlier disclosure, W. T. Miller, Jr., “Preparation and Technology of Fluorine and Organic Fluorine Compounds”, National Nuclear Energy Series, VII-I, Chapter 32 (pp. 567-685), the pyrolysis of TFE is carried out using a nickel tube, 2.5 cm I.D., and heated along a 12 in (30.5 cm) length to a temperature of 435-750° C. (p. 592). U.S. Pat. No. 2,970,176 discloses the carrying out of the TFE pyrolysis reaction at temperatures of 700° C. to 900° C., but wherein higher boiling perfluoroolefins than TFE are co-fed to the reactor tube. The reactor tube is a ½ in diameter 15 ft long stainless steel pipe arranged in loops (helical coil). These patents are followed by U.S. Pat. No. 3,446,858 which discloses carrying out the same pyrolysis reaction but at atmospheric pressure, optionally in the presence of octafluorocyclobutane, by adding superheated steam to the reaction zone. This patent recognizes that stainless steel is not sufficiently inert to the reaction products and uses a tubular reactor of fused silica 22.5 mm long. Unfortunately, fused silica is not suitable for making a commercial size tubular reactor which typically exceeds 50 ft (15.24 m) in length and has a nominal inner diameter of ¾ in (1.9 cm); such reactors are pipes having a 1.05 in (2.67 cm) OD and an ID of 0.824 in (sch 40) or 0.742 in (sch 80), corresponding to IDs of 2.09 cm and 1.88 cm, respectively. Such a fused silica reactor though suitable for laboratory work is too fragile to be used as a material of construction for long commercial reactors, especially when coiling is desired to minimize plant space occupied by the reactor.
U.S. Pat. No. 3,873,630 discloses pyrolysis of TFE to HFP using CO2 as a co-feed, wherein the tubular reactor is made of Inconel® 600 alloy (nickel-chromium (at least 13 wt %) alloy with a small amount of silicon and possibly iron). Inconel® 600 alloy has become the choice for material of construction of the tubular reactor in the TFE pyrolysis reaction because it is relatively inert under the pyrolysis reaction conditions when operated at temperatures no greater than 825° C. Beside being inert, fabricated tubes of the alloy can be welded end-to-end and are ductile so that they can be coiled to form commercial size reactors.
It has been found that operating the Inconel® 600 alloy reactor at temperatures higher than 825° C., e.g. at 830° C., greatly reduces the amount of time that the pyrolysis reaction can be conducted, before the reactor has to be shut down for repair. At the higher operating (pyrolysis) temperature, cracks appear in the wall of the reactor, extending through the entire thickness of the reactor wall. The formation of these cracks occur during the operation of the reactor, sometimes coinciding with an operation incident that causes a sudden increase in stress imposed upon the reactor, as occurs with a sudden change in reactant feed to the reactor, a sudden change in temperature, and/or a mechanical disturbance caused by a shifting of the reactor. This formation of one or more cracks in the reactor wall enables reaction products to escape from the side of the reactor, rather than from the exit end of the reactor for separation and recovery of the HFP, unreacted TFE, and treatment of undesired reaction products, such as perfluoroisobutylene, (CF3)2C═CF2 (PFIB), which is toxic. Operation of the reactor at a temperature no higher than 825° C., while providing long HFP production runs between reactor shutdowns, has the disadvantage of a loss in productivity, i.e. less HFP is produced by the reactor.